carried out in these cases and the extent of chemical
quenching with respect to the overall (chemical + physical)
quenching was very small. The solvent water appeared to
play an essential role as no evidence for the electron-transfer
mechanism was found in MeCN.
conversion of starting amine ranging from 8.4 to 35%)
indicating that, particularly with 1a, chemical quenching of
1O2 is an important fraction of the overall quenching.
Products, yields (with respect to the amount of 1O2 generated
by the endoperoxide),8 and fractions of chemical quenching
(Qc) are reported in Table 1 together with the rates of total
Also in view of the scarcity of detailed product studies
for the reactions of amines with 1O2,1d we have now extended
our investigation to silylamines. These species have very low
oxidation potentials (ca. 0.4 V lower than those of the
corresponding amines. See Supporting Information) and
easily form radical cations that undergo a very fast, nucleo-
philically assisted, cleavage of the C-Si bond.4 Thus, we
felt that these properties might significantly influence the
1
Table 1. Oxygenation of Amines by O2
productsa (yield, %)
subst
solvent
kQ (M-1 s-1
1.5 × 108
)
2
3
CH2O Qcb (%)
1a MeCNc
19
24
2.1
20
g
21
44
1
extent of chemical quenching in the reaction with O2 as
20% MeOHc 1.2 × 108
26
well as the mechanism of product formation. On this basis,
we have carried out a kinetic and products study of the
1b MeCNd
9.4 × 108
1.2
5.6
h
1.9
i
6
1.2
7.5
20% MeOHe 5.0 × 108
1
quenching of O2 in MeCN by N-methyl-N-(trimethylsilyl-
methyl)cyclohexylamine (1a) and N-methyl-N-(trimethylsi-
lylmethyl)-p-toluidine (1b) (Scheme 2). In this paper, we
4
5
MeCNe
2.65 × 108
6.6
11
3.0
j
7
12
9.6
11
20% MeOHe 1.5 × 108
MeCNd
20% MeOHd 6.2 × 108
1.0 × 109f
h
h
h
h
0
0
Scheme 2
a Referred to the amount of 1O2 produced by the endoperoxide and
determined by GC analysis (error ( 5%) except for CH2O that was
spectrophotometrically determined after treatment with the Nash reagent
(error ( 5%). b Sum of the yields of 2 and 3 vs 1O2. c Endoperoxide/amine
) 1:1. d Endoperoxide/amine ) 10:1. e Endoperoxide/amine ) 2:1. f Ref-
erence 3e. g Product below the detection limit (3%). h Product below the
detection limit (0.01%). i Product below the detection limit (0.3%). j Product
below the detection limit (0.05%).
quenching, kQ (physical plus chemical), measured by time-
resolved luminescence at 1270 nm. For comparison purposes,
in Table 1 are displayed the data for the corresponding
nonsilylated amines, N,N-dimethylcyclohexylamine (4) and
N,N-dimethyl-p-toluidine (5).
wish to present the results of these investigations that suggest
that these reactions involve an electron-transfer step.
The reactions of 1a and 1b with 1O2, thermally generated
by 1,4-dimethylnaphthalene endoperoxide,5 were carried out
in dry MeCN6 and in MeCN containing 20% (v/v) of
methanol (henceforth referred to as MeCN-MeOH, for the
sake of brevity). Using substrate 0.01 M and endoperoxide
from 0.01 to 0.1 M (depending on the extent of chemical
quenching),7 significant product yields were observed (the
Considering first the kinetic results, it appears clear that
in both MeCN and MeCN-MeOH the rate of quenching kQ
is very little affected by replacing H by the TMS group,
which suggests that formation of the encounter complex, or
exciplex, plays a kinetically important role with both amines
and silylamines.1a Since the quenching rates of 4 and 5 are
slightly higher than those of the corresponding silylamines,
steric effect by TMS affecting the formation of the exciplex
intermediate can be suggested.9 Probably, the effect is larger
than that inferred by the rate data in Table 1 since, as already
said, the presence of the TMS group lowers the oxidation
potential of the amine and this is expected to lead to an
(3) (a) Peters, G.; Rodgers, M. A. J. Biochim. Biophys. Acta 1981, 637,
43. (b) Manring, L. E.; Foote, C. S. J. Phys. Chem. 1982, 86, 1257. (c)
Saito, I.; Matsuura, T.; Inoue, K. J. Am. Chem. Soc. 1983, 105, 3200. (d)
Haugen, C. M.; Bergmark, W. R.; Whitten, D. G. J. Am. Chem. Soc. 1992,
114, 10293. (e) Darmanyan A. P.; Jenks, W. S.; Jardon, P. J. Phys. Chem.
A 1998, 102, 7420. (f) Bernstein, R.; Foote, C. S. J. Phys. Chem. A 1999,
103, 7244. (g) Cocquet, G.; Rool, P.; Ferroud, C. J. Chem. Soc., Perkin
Trans. 1 2000, 14, 2277.
(4) (a) Hasegawa, E.; Xu, W.; Mariano, P. S.; Yoon, U.-C.; Kim, J.-U.
J. Am. Chem. Soc. 1988, 110, 8099. (b) Zhang, X.-M.; Mariano, P. S. J.
Org. Chem. 1991, 56, 1655. (c) Su, Z.; Mariano, P. S.; Falvey, D. E.; Yoon,
U. C.; Oh, S. W. J. Am. Chem. Soc. 1998, 120, 10676. (d) Gould, I. R.;
Godleski, S. A.; Zielinski, P. A.; Farid, S. Can. J. Chem. 2003, 81, 777.
(5) (a) Turro, N. J.; Chow, M. F. J. Am. Chem. Soc. 1981, 103, 7218.
(b) Adam, W.; Prein, M. Acc. Chem. Res. 1996, 29, 275. (c) Greer, A.;
Vassilikogiannakis, G.; Lee, K.-C.; Koffas, T. S.; Nahm, K.; Foote, C. S.
J. Org. Chem. 2000, 65, 6876. (d) Ben-Shabat, S.; Itagaki, Y.; Jockusch,
S.; Sparrow, J. R.; Turro, N. J.; Nakanishi, K. Angew. Chem., Int. Ed. 2002,
41, 814. (e) Poon, T.; Turro, N. J.; Chapman, J.; Lakshminarasimhan, P.;
Lei, X.; Jockusch, S.; Franz, R.; Washington, I.; Adam, W.; Bosio, S. G.
Org. Lett. 2003, 5, 4951.
(8) (a) The yield of singlet oxygen generated from 1,4-dimethylnaph-
thalene endoperoxide in MeCN and in MeCN-MeOH at T ) 40 °C was
spectrophotometrically measured, according to a literature procedure,3c,5a
using 1,3-diphenylisobenzofurane as singlet oxygen acceptor. It resulted to
be 70% (in MeCN) and 80% (in MeCN-MeOH) vs the initial amount of
endoperoxide, values very close to that previously observed in dioxane.5a
These values are much larger than that (25%) reported by Gu¨nther et al.8b
for the solvent MeCN at 20 °C, which was used in our previous work to
calculate the chemical quenching of N,N-dimethylbenzylamine.2 On the
basis of the present value in MeCN, the actual chemical quenching of
N,N-dimethylbenzylamine in MeCN is 3% and not 9% as reported. (b)
Gu¨nther, G. S.; Lemp, E. M.; Zanocco, A. L. J. Photochem. Photobiol. A
2002, 151, 1.
(6) Carefully dried by reflux on CaH2.
1
(7) Under these conditions all the O2 (kd ca. 2 × 104 s-1 in MeCN)1b
generated is quenched by the substrate (0.01 M, kQ ca. 108 M-1s-1).
(9) Monroe, B. M. J. Phys. Chem. 1977, 81, 1861.
1784
Org. Lett., Vol. 8, No. 9, 2006